The embodiments herein relate generally to geothermal well loops in a subterranean formation and, more particularly, to grout fluids for use in grouting the annulus between the geothermal well loop and the subterranean formation.
Geothermal well loops (or simply “well loops”) provide an energy-efficient, cost-effective, and environmentally friendly heating and cooling systems that transfer heat to and from the ground. Since the Earth's subsurface is at a near constant temperature year round, it is an efficient heat exchange medium. In a vertical closed well loop system, two pipes joined by a U-shaped connector at the bottom, forming a continuous tubular, are placed vertically in a wellbore drilled in a subterranean formation. This type of system is generally used for heating and cooling residential and commercial buildings. A thermally enhanced grout that is clay-based, typically bentonite-based, and is pumped into a wellbore to fill the annular space between the tubular and the formation. The grout forms a seal to prevent contamination of the subsurface from the surface, as well as preventing groundwater contamination. The grout fluids may further include conductive materials to aid in transferring temperature between the well loop and the Earth. For example, a carrier fluid may be circulated through the well loop to transfer heat to and from a heat exchanger at the surface. In the winter, the fluid collects heat from the ground and carries it to the heat exchanger at the surface, which provides heat to a house or building. The process is reversed in the summer to remove heat from the surface and place it in the ground, thus cooling the house or building.
Grout fluids have a high solids content (e.g., about 35 to about 72% total solids by weight. However, such grout fluids must be easily flowable at the surface and able to set (i.e., cure) after being placed into a wellbore. Once the grout is set, it must be able to suspend the conductive materials as well have a low permeability to provide an effective seal and to prevent water-phase separation.
In traditional applications, a clay (e.g., bentonite) is added to water and mixed to ensure hydration in a tank (e.g., with a paddle mixer). The conductive materials are added next, and pumping begins once all of the conductive materials have been added. The products are mixed and pumped quickly to ensure that the grout does not set up in the mixing tank and plug the system. A dual piston pump may be used to pump the grout into a wellbore through a tremie line. As used herein, the term “tremie” refers to a tubular, such as a pipe, through which a grout fluid is placed into a wellbore. The term “tremie” as used herein is not limited to grout fluid placement at a particular water level and use of a tremie to place grout fluid may be performed below or above water level, without departing from the scope of the present disclosure. A piston pump may be used because of its ability to pump materials with a high solids content at higher pressures. The tremie is then slowly retracted back to the surface with the bottom of the line staying submerged in the grout to prevent formation of cavities in the grout column.